In digital data transmission systems, data in binary form is transmitted over media such as wires or fiber optic cables from a transmission line transmitter to a transmission line receiver. The binary data waveform is degraded with respect to its timing and its amplitude as it propagates along the transmission media due to electrical noise and dispersion. This degradation of bits in the binary waveform results in incorrect interpretations by the receiver of bits sent by the transmitter across the transmission media. The incorrect interpretations are referred to as bit errors and the rate at which they occur within a communications network is referred to as a bit error rate.
In addition to variances in amplitude, the actual received data transmissions may be displaced in time from the true transmission. This time displacement, or intersymbol interference (ISI), of the transitions is caused by a new wave arriving at the receiver before the previous wave has reached its final value. Intersymbol interference (ISI) occurs due to pulse spreading caused by the dispersion of the transmission media. Variations in the clock rate and phase degradations (jitter) also distort the zero crossings resulting in decision time misalignment. When a pulse is transmitted in a given time slot, most of the pulse energy will arrive in the corresponding time slot at the receiver. However, because of this pulse spreading induced by the transmission medium, some of the pulse energy will progressively spread into adjacent time slots resulting in an interfering signal.
The transmission line receiver typically includes a regenerative repeater for reconstructing the transmitted data, regardless of electrical noise and transmission media degradations. The data pulse train is thereby permitted to travel through a dispersive and noisy medium, being repeatedly reconstructed at each repeater location to prevent degradation of the bits to the point where they are unrecognizable. In this manner, the bits remain impervious to most of the degradation introduced by the transmission medium, thereby reducing the bit error rate associated with the communications network. In the case of long distance, high capacity digital systems, the accuracy of the regenerative repeaters will often determine the overall effectiveness of the system.
Reconstruction of the originally transmitted signal by the repeater can be achieved by sampling the pulse train at a regular frequency equal to the bit rate, and at each sample instant making a decision of the most probable symbol being transmitted. Typically, a threshold level is chosen to which the received signal is compared. Above this threshold level a binary one is registered, and below the threshold a binary zero is registered. The regenerator circuit makes these zero or one decisions at times corresponding to the center of the bit intervals based on the clock information provided by a timing circuit. The center of the bit intervals generally correspond to the mid-points of the threshold level crossings of the pulse train. By setting the decision times midway between the threshold level crossings, the odds of accurately reconstructing the transmitted bit are increased, thereby decreasing the bit error rate.
Data communications networks typically include a link monitoring mechanism for measuring the bit error rate of the received data. The link monitoring usually is implemented via complex software which compares the received symbol codes with correct symbol codes to detect the bit errors. Because the bit error rate is low for most communications systems, given the state of the art and the presence of regenerative repeaters, a significant amount of time must pass before enough bit errors are accumulated to permit determination of an average bit error rate. However, it is desirable to detect an unacceptably high bit error rate as quickly as possible in order to permit re-routing of the binary data stream over an alternative communications link before a large number of errors are transmitted. In this manner, less data needs to be re-transmitted, thereby increasing the performance of the communications link.
It is an object of the present invention, therefore, to provide a high-speed, low-power mechanism for quickly estimating the bit error rate of a communications link to enhance the overall efficiency of the communications link.